1. Technical Field
Dental prostheses and apparatus and methods of manufacturing them. In particular, a dental crowns and bridges, and an associated CAD-CAM method for making the crowns and bridges.
2. Description of Related Art
Dentures, partial dentures, dental crowns, and dental bridges are prosthetic articles that are made to replace some or all of a person's natural teeth. A dental crown is a rigid fitting that completely encloses or caps a tooth or multiple teeth. A dental bridge is an artificial tooth or teeth that replaces missing natural teeth and that is joined to adjacent remaining teeth.
When a dentist diagnoses that his/her patient has a need for a dental crown or bridge, he/she will generally prepare the tooth (or teeth in the case of a bridge) to receive the crown or bridge by grinding the tooth with a suitable tool such as a high speed rotating diamond bur (also spelled “burr” in some sources). The bur is used to remove tooth structure that is decayed or to provide space that will be occupied by the crown or bridge. The dentist then obtains an accurate impression of the patient's existing gums and prepared teeth, including the tooth or teeth that will receive the crown or bridge, by using a silicone elastomer such as polyvinyl siloxane. The impression is then sent to a dental laboratory that specializes in manufacturing custom dental prosthetics for their customers who are dentists or other dental laboratories.
The field of fixed dental prosthetics, such as crowns and bridges, has evolved rapidly. In the 1960's, the standard of care was a porcelain-fused-to-metal (PFM) type crown. The porcelain used was typically feldspathic, i.e., made from feldspar mineral, and the metals used were gold-based alloys. A problem with the conventional feldspathic PFM is that the porcelain is very durable and tends to wear opposing natural dentition. In addition, metal can show through around the marginal edges of the porcelain, resulting in there being the perceived appearance of “black” lines at the margins.
In the 1980's, newer classes of porcelains were developed that were “low-fusing,” which were “softer” than conventional feldspathic porcelains. The advantage of these porcelains was that the material was less abrasive to opposing natural dentition.
In the 1990's, all-ceramic fixed restorations were introduced. The advantage of this class of restorations was improved aesthetics, since no metal was used underneath the porcelain. The problem with many of these all-ceramic products was that they lacked strength, especially needed in the posterior teeth. Crowns made by computer aided design (CAD) and computer aided manufacturing (CAM) were also introduced. This approach reduced the expertise and labor costs required for conventional crowns.
In the late 1990's, fixed restorations were also made with an alumina substructure and overlaid with porcelain. This approach solved the problem with potential exposure of metal at the margins while also providing the crown or bridge with the required strength.
In the 2000's, zirconia was introduced as a substructure because it provided increased strength that was sufficient for posterior applications. Porcelain was still used to overlay the zirconia because the base color of zirconia at that time was white. The cost of milling zirconia by CAD-CAM based tooling was relatively expensive at the initial stage. However, the cost of milling equipment and software continued to decrease, such that more dental laboratories started using zirconia as the primary substructure for crowns. The primary problem with zirconia is the inherent color: it is bright white and does not have the appearance of natural teeth. Since most teeth are not completely white, some skill and cost is required to overlay porcelain on the zirconia in order to deliver an esthetic natural appearing tooth.
In the 2010's, zirconia has evolved and is now available in more translucent colors, in addition to more natural-looking tooth shades. Most recently, zirconia is being used for “full-contour” crowns, especially in the posterior region. “Full-contour” means that a porcelain overlay is not required on top of the zirconia. This evolution means that very little direct labor is required to make the crown. The only labor required is the person(s) executing the software program, supervising the automated milling operation, and finishing/inspecting the resulting crown product.
Many dental laboratories have embraced this approach because of the low production costs. However, certain problems remain to be solved with this manufacturing method and/or with the dental prostheses made by it. One remaining problem is that the zirconia is very abrasive to the opposing dentition, which was the original problem with conventional feldspathic PFMs. One countermeasure is to use the shaded or translucent zirconia as the base material, and then place “low-fusing” porcelain on top of the zirconia. The procedure is performed by dental labs today, especially in the anterior (front) teeth to give the appearance of multi-layers which enhance esthetics. However, a problem with this approach is that it requires skills and labor to layer the porcelain by hand, which makes the crown more expensive to produce.
Thus there remains a need for a method of making a dental crown or bridge that is of low cost, that uses materials resistant to wear and having a desired aesthetic appearance, and that is amenable to automation via the use of CAD-CAM software and CAD-CAM operated machines.